Contact: Jane Platt

September 17, 1999

Onward to Io!

Onward to Io! That's the rallying cry now that NASA's Galileo spacecraft has successfully completed its fourth and final flyby of Jupiter's pockmarked moon, Callisto. The Callisto encounters were designed to lower Galileo's orbit to bring it closer to the fiery moon Io, the most volcanic body in the solar system. Galileo will fly by Io in October and then again in November.

The spacecraft dipped down to 1,052 kilometers (654 miles) above Callisto's surface Thursday, September 16 at 11:02 a.m. Pacific Daylight Time. The spacecraft is operating normally, has completed all planned recording, and is now playing back science information gathered during the flyby by the instruments that study magnetic fields and particles. That information was stored on Galileo's onboard tape recorder.

In an expected repeat of an occurrence from previous orbits, a minor glitch popped up at 3:39 p.m. PDT on Wednesday, with a computer reset of the non-spinning portion of the spacecraft. Onboard software handled the problem correctly, and recording of data continued without interruption.

Currently, Galileo engineers are trying to determine why the spacecraft's ultraviolet spectrometer instrument began malfunctioning during the previous flyby of Callisto in August. They sent a preliminary set of commands to the instrument designed to re-start its microprocessor, which apparently had stopped working. Preliminary results seem to confirm that the microprocessor was restarted successfully for this week's encounter. Engineers will conduct further analysis of the data gathered during this flyby to help assure that the ultraviolet instrument is operating normally. The instrument studies Jupiter's atmosphere and aurora, the surfaces and atmospheres of its moons, and the doughnut-shaped cloud of charged plasma particles in the orbit of the volcanic moon Io.

Galileo has been orbiting Jupiter and its moons since December 1995. The spacecraft is approaching the grand finale of its two-year extended Galileo Europa Mission, a follow-on to the primary mission that ended in December 1997. JPL, a division of the California Institute of Technology, Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.